1. Field of the Invention
The present invention relates to a plasma etching method for preferentially etching a silicon nitride film (SiNx) arranged on a surface of a target object, such as a semiconductor wafer.
2. Discussion of the Background
In manufacturing processes of semiconductor devices, such as ICs and LSIs, predetermined circuit patterns are formed by subjecting a semiconductor wafer to film formation by means of, e.g., CVD (Chemical Vapor Deposition), and patterning by means of, e.g., etching. There is plasma dry etching as a representative of the etching processes. In plasma dry etching, a patterned photo-resist film is used as a mask and the exposed portion of an objective film is etched by an etching gas which has been turned into plasma.
In plasma dry etching, it is important to set the selectivity of an objective film to be etched, relative to a different kind of film not to be etched, to be as high as possible. Conventionally, a mixture gas of CHF3 gas and O2 gas is used to preferentially etch a silicon nitride film (SiNx) relative to a silicon oxide film (SiO2). This mixture gas is used along with Ar gas, which is a carrier gas. Where the mixture gas of CHF3 gas and O2 gas is used, the selectivity of SiNx relative to SiO2, i.e., (an SiNx etching rate) (an SiO2 etching rate) is about two at most. In this case, the selectivity of SiNx relative to Si, i.e., (an SiNx etching rate) (an Si etching rate) is also about two at most.
Where requirements on line widths and hole diameters are not so strict, a selectivity of about two does not cause serious problems. However, where the line widths and hole diameters are required to be of the sub-micron order, with an increase in the density and integrity degree of semiconductor devices, a selectivity of about two cannot satisfy the requirements.
Jpn. Pat. Appln. KOKAI Publication No. 8-59215 (U.S. Ser. No. 08/189027, now abandoned) discloses an etching method which uses a mixture gas of CHxF4xe2x88x92x (where x is 2 to 3,) and one or more oxygen-containing gases selected from a group consisting of O2, CO and CO2, in order to preferentially etch silicon nitride relative to silicon oxide, metal silicide, or silicon. In this etching method, preferably, the CHxF4xe2x88x92x consists essentially of CHF3, the oxygen-containing gas consists essentially of CO2, or both of them, and more preferably, CO2. As a result, this publication states that this etching method improves the selectivity, i.e., (an SiNx etching rate)/(an SiO2 etching rated), to be about four.
On the other hand, in plasma dry etching, it is important to keep the planar uniformity of etching high, the planar uniformity being defined by the difference between processed sizes at the center and the periphery of a target substrate. For example, it can be said that the planar uniformity is preferable where the difference between processed sizes at the center and the periphery falls within a range of xc2x17%, and more preferably within a range of xc2x15%. The selectivity and the planar uniformity have a trade-off relationship in terms of a certain parameter. Jpn. Pat. Appln. KOKAI Publication No. 8-59215 described above states a summary account about the selectivity, but does not at all consider the planar uniformity.
Accordingly, it is an object of the present invention to provide a plasma etching method which can present a good balance between the etching selectivity and the etching planar uniformity of silicon nitride relative to silicon oxide and silicon.
In particular, the present invention provides a method of plasma-etching a target substrate in a plasma etching apparatus, the target substrate having a surface on which a first layer consisting essentially of silicon nitride and a second layer consisting essentially of silicon or silicon oxide are arranged, which entails:
a) setting the etching apparatus in light of a set value of etching selectivity of the first layer relative to the second layer, and a set value of etching planar uniformity defined by a difference between sizes to be processed at a center and a periphery of the target substrate;
b) placing the target substrate in a process chamber of the etching apparatus;
c) supplying a process gas into the process chamber, while exhausting the process chamber, the process gas comprising a mixture gas of CH2F2 gas and O2 gas at a ratio of from 55% to 100% in volume percentage, the CH2F2 gas and the O2 gas being mixed at a first mixture ratio (CH2F2/O2) of from 0.2 to 0.6 in volume ratio;
d) turning the process gas into plasma by electric discharge, while keeping the process chamber at a first pressure of from 50 mTorr to 100 mTorr; and
e) etching the first layer preferentially to the second layer with the plasma; and
f) wherein parameters for the planar uniformity, by which the etching apparatus is set in light of the set value of the planar uniformity, include the first pressure and the first mixture ratio, and as the set value of the planar uniformity is more strict, either one of the first pressure and the first mixture ratio is set higher.